Field of the Invention
This invention relates to an automatic transmission, and more specifically to an automatic transmission which operates properly even when an electrical fault or problem occurs in any of its solenoid valves.
Description of the Related Art
In all conventional automatic transmissions, rotation produced by an engine is transmitted to a shift mechanism via a hydraulic power transmission so that different speeds are achieved by the shift mechanism. The shift mechanism is provided with a plurality of planetary gear units. Elements such as ring gears, carriers and sun gears, which make up each planetary gear unit, are selectively engaged or released by frictional engagement elements to achieve the different speeds.
A hydraulic circuit is provided to engage or release the respective frictional engagement elements. Oil at a regulated pressure is fed to or drained from a hydraulic servomotor corresponding to each frictional engagement element, whereby the frictional engagement element is either engaged or released at a predetermined timing.
Further, conventional automatic transmissions are each provided with "P" (parking), "R" (reverse), "N" (neutral), "D" (drive), "S" (second) and "L" (low) ranges so that, once a desired one of the "D" "S" and "L" ranges is selected by a driver, shifting can then be performed automatically.
In a 4-forward/1-reverse speed transmission, for example, shifting is automatically performed among the 1st speed to the 4th speed when the "D" range is selected by the driver, among the 1st speed to the 3rd speed when the "S" range is selected by the driver, and between the 1st speed and the 2nd speed when the "L" range is selected, all depending, for example, on vehicle speed, throttle position and various other running conditions.
Selection of each range in accordance with the driver's desire is conducted by moving a shift lever to one of the range positions arranged in a straight line, i.e. the so-called "I" pattern.
The above-described automatic transmission, however, does not permit free manual selection of any speed, so that the feel of the shifting is not satisfactory. However, a transmission has been disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 2-8545, in which a shift lever is moved in a gear shift path to bring it into contact with one of plural sensors corresponding to different speeds and the desired speed is achieved in accordance with an electrical signal from the one sensor. Another transmission has been disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 61-157855 which is provided with a shift pattern which includes shift positions arranged in a so-called "H pattern," in addition to the range positions employed in conventional automatic transmissions and arranged in the "I" pattern, whereby a driver is allowed to select either running in an automatic shifting mode or running in a manual shifting mode.
In automatic transmissions of the above-described type, it is possible to run in a manual shifting mode, for example, at the 1st to 4th forward and one reverse speeds, respectively, by moving a shift lever to one of the corresponding shift positions in the "H" pattern.
In the conventional automatic transmissions described above, the position of a range pressure producing means, namely, a manual valve, determines whether an engine brake is effectively applied or not. At the "D" range position, for example, it is impossible to effectively apply an engine brake responsive to running conditions.
The foregoing led to provision of a coast brake cutoff valve, for the selective application of an engine brake, between the manual valve and shift valves with the coast brake cutoff valve controlled by a solenoid valve. Such an arrangement made it possible to effectively apply an engine brake at the 1st and 2nd speeds regardless of the position of the manual valve, in other words, regardless of range pressure which is produced.
In an automatic transmission, various valves are controlled by selectively turning on/off their corresponding solenoid valves. Therefore it becomes impossible to perform a gear shift if an electrical problem occurs in the corresponding solenoid valve or valves. The automatic transmission is accordingly designed to be able to achieve three forward and one reverse speeds by controlling the manual valve even if one or more of the solenoid valves develop an electrical problem.
In the automatic transmission with the coast brake cutoff valve arranged between the manual valve and the shift valves, however, the coast brake cutoff valve can be changed over by on/off controlling an associated solenoid valve. If an electrical problem occurs in the associated solenoid valve, the manual valve and the shift valves are cut off from each other so that it becomes no longer possible to achieve the three forward and one reverse speeds, even when the manual valve is in one of positions "D", "S" and "L".
Further, development of any electrical problem with the solenoid valve controlling the coast brake cutoff valve makes it impossible to effectively apply an engine brake at the 1st and 2nd speeds.
An object of this invention is to overcome the above-described problems of the conventional automatic transmissions, and therefore to provide an automatic transmission in which corresponding engine brake or brakes can be selectively and effectively applied no matter which range pressure is produced by a range pressure producing means and, even if an electrical problem occurs in one or more solenoid valves, a necessary speed can still be achieved and necessary engine brake or brakes can still be effectively applied.
An automatic transmission according to the present invention is therefore provided with means for producing a range pressure in accordance with each position of the shift lever and also with a controller for outputting a shifting output signal in accordance with the position of the shift lever.
The automatic transmission is also provided with shifting solenoid valves which are switched between ON and OFF responsive to the shifting output signal from the controller, whereby shift valves are selectively changed over. A coast brake cutoff valve is arranged between the shift valves and the range pressure producing means and is capable of selectively feeding a hydraulic pressure to hydraulic engine brake servomotors.
The automatic transmission is further provided with an engine brake solenoid valve which is switched between ON and OFF responsive to the shifting output signal from the controller, whereby the coast brake cutoff valve is selectively switched. A lockout valve is arranged between the coast brake cutoff valve and the range pressure producing means and is capable of selectively feeding the range pressure to the coast brake cutoff valve.
The lockout valve is connected to one of the shifting solenoid valves and, upon occurrence of a problem in the shifting solenoid valves or the engine brake solenoid valve, feeds the range pressure as a signal hydraulic pressure to the coast brake cutoff valve so that a hydraulic pressure is selectively fed to the hydraulic engine brake servomotors via the coast brake cutoff valve.
As described above, the automatic transmission according to the present invention is provided with means for producing a range pressure in accordance with the position of the shift lever and is also provided with a controller for outputting a shifting output signal in accordance with the position of the shift lever.
The shifting solenoid valves, which are switched ON and OFF responsive to the shifting output signal from the controller, control the shift valves. It is therefore possible to achieve a desired speed in accordance with the position of the shift lever.
Arranged between the shift valves and the range pressure producing means is the coast brake cutoff valve which selectively feeds a hydraulic pressure to the hydraulic engine brake servomotors. The coast brake cutoff valve is changed over by switching ON and OFF the engine brake solenoid valve in accordance with a shifting output signal from the controller. When the engine brake solenoid valve is turned on, it is possible to effectively apply a desired engine brake by feeding a hydraulic pressure to the corresponding hydraulic engine brake servomotor via the coast brake cutoff valve and the corresponding shift valve.
The lockout valve is arranged between the coast brake cutoff valve and the range pressure producing means. The lockout valve is connected to one of the shifting solenoid valves so that, even if an electrical problem occurs in any one of the solenoid valves, the automatic transmission is still allowed to operate properly. This is hereinafter referred to as the "fail safe feature". The range pressure is fed, as a signal hydraulic pressure to the coast brake cutoff valve, whereby a hydraulic pressure is fed to the desired hydraulic engine brake servomotor via the coast brake cutoff valve.
Therefore an engine brake can still be applied effectively even if the vehicle is running in the fail safe mode.